It is hard to get a simple answer on that. Mathematically an orbit is fixed, but in practice it depends on the orbital radius. The lower orbits still encounter runaway atmospheric particles and space dust so the lower the orbit the more adjustment is needed to maintain the orbit.
What I know by heart is that the ISS travels at ~17,500 mph or 7.7 km/s and it falls up to 1.5-2 kilometers per month. so every day it falls up to 100meters. To prevent this, the ISS engines are fired every month with the engines pointed retrograde ( to increase the tangential velocity ). This is enough to speed it up. Due to its sheer mass, the Space Shuttle was used to boost it up with the extra fuel it had before leaving every time, but since then this maintenance is done solely by the Progress Module ( a Russian module - basically a Soyuz, which also acts as the lifeboat of the ISS ).
And the fuel spent is a lot ( here documenting comes in) :

"The ISS requires an average 7,000 kg of propellant each year for altitude maintenance, debris avoidance and attitude control."
and
"Multiple supply vehicles are required to satisfy the ISS's 7,000 kg annual average propellant need.
The Propulsion Module would hold 9808 kg fuel. Progress M holds 1100 kg; Progress M1 holds 1950 kg. ESA ATV holds 4,000 kg."
Wow this is much more than I thought.

Update1:This bandwidth limit is killing me, but I will retry until it works :)
Update2:Since I coudn't post until now I remembered another image which rocked my world:

Finally :)